Ignition Switch Wiring

by James W. Hebert

Most outboard motors are operated and started using a key ignition switch. The switch configuration and wiring is fairly standardized. This description is specific to OMC outboard motors but will be typical of most.

Ignition Switch Wiring

The ignition switch on most outboard motors is operated by a key, much like used traditionally in vehicles. The key affords some measure of security to engine operation and starting. The switch generally has three positions of rotation which are typically given the legends OFF (most counter-clockwise), ON or RUN, and START (most-clockwise). There may also be an additional switch operated from pushing inward on the key, denoted as CHOKE or PRIME. The rotating switch is used to control the engine and starter motor, while the push switch generally operates a priming circuit to aid in engine starting. The terminal identifiers used on the switch follow a historical legend, and standarized color coding is used on the conductors to aid in identification of their function.

There are typically three separate switches in the ignition switch assembly. One switch feeds the battery voltage to several destinations as the switch is rotated. A second switch provides the battery voltage to the choke when the switch is pushed. A third switch operates only in the OFF position.

The color coding of the conductors varies slightly by manufacturer and is documented in a separate article. The color coding described here follows the practice of OMC.

The arrangement of contacts in the switch will typically produce the following connections as the switch is rotated and pushed:

OFF Position

The two M terminals are the poles of an isolated switch which are only connected in the OFF position. These terminals are used to connect the ignition kill circuit (or the magneto as it was historically called and hence the "M" designation) to ground. The lower M terminal is connected to battery negative or ground (black), and the raised M terminal is connected to the ignition kill circuit (Black with Yellow stripe). It is important to observe this convention because the voltage on the wire coming from the engine can be quite high, 300-volts or more. This circuit goes to the raised terminal which helps isolate it from the other connections. The M terminals may also be used as a wiring point for connecting another switch in parallel. This second switch is typically operated by a safety lanyard. When the second switch operates, it also connects the two M terminals together, killing the engine, even if the ignition switch is not in the OFF position. See the wiring pictorial below.

ON Position

The B terminal with +12-volts from the battery is connected to the A (or I) terminal, supplying battery voltage to all the accessory loads and to the engine ignition. This is the ON or RUN position.

START Position

The B terminal is connected to the A and S terminals, supplying battery voltage to the engine ignition and accessory loads, and to the engine starter solenoid. This is the START position.

PRIME in ON or START Position

In either the ON or START positions of the rotating switch, the push switch will connect the B terminal to the C terminal, supplying battery voltage to the engine primer circuit. The B terminal is also connected to the A (or I) terminal in RUN and to the A (or I) and S terminal in START.

Overall Switch Wiring Pictorial

The ignition switch is often integrated with several other devices, typically a lanyard switch (often called a kill switch), an alarm buzzer, a safety switch which operates when the remote shift control is in the NEUTRAL position, and a connector for adding a tachometer. This is shown below.

It is very common that the A (or I) terminal also feed switched battery voltage to many other loads, such as gauges or other vessel instrumentation associated with the engine, and an engine hour meter. There may be several other devices connected to the A (or I) terminal of the ignition switch. The aggregate load of these devices must not exceed the current rating of the fuse which protects this circuit branch. The fuse is typically located under the engine cowling at the point where the RED with VIOLET stripe conductor branches from the engine's primary battery wiring, often at the starter solenoid terminal.

Red Amphenol Connector Wiring Pictorial

At the engine, the standard OMC wiring harness mates to the engine harness using a large circular Amphenol red rubber-body connector. The pin arrangement, conductor color code, and circuit functions of the connector on the engine wiring harness are shown in the drawing below.

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Last modified: Thursday, 28-Mar-2013 10:50:34 EST
Author: James W. Hebert
This article first appeared December 2, 2006.